SMB: Implement encryption, cleanup RequireSignature

[gpotter2]

This PR:

• adds SMB encryption.
• cleans up how and when signature is required by the client and server, failing in cases where it should fail.

[codecov]

Codecov Report

Attention: Patch coverage is 86.11111% with 25 lines in your changes missing coverage. Please review.

Project coverage is 81.67%. Comparing base (92925da) to head (4d98eb5).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
scapy/layers/smb2.py	90.42%	9 Missing ⚠️
scapy/layers/smbclient.py	72.41%	8 Missing ⚠️
scapy/layers/smbserver.py	86.00%	7 Missing ⚠️
scapy/layers/ntlm.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4643      +/-   ##
==========================================
+ Coverage   80.71%   81.67%   +0.95%     
==========================================
  Files         359      359              
  Lines       86052    86378     +326     
==========================================
+ Hits        69461    70545    +1084     
+ Misses      16591    15833     -758     

Files with missing lines	Coverage Δ
scapy/layers/smb.py	76.70% <100.00%> (+0.33%)	⬆️
scapy/layers/ntlm.py	87.31% <50.00%> (+0.02%)	⬆️
scapy/layers/smbserver.py	72.81% <86.00%> (+1.84%)	⬆️
scapy/layers/smbclient.py	74.02% <72.41%> (-0.37%)	⬇️
scapy/layers/smb2.py	88.19% <90.42%> (+0.90%)	⬆️

... and 28 files with indirect coverage changes